The present invention relates generally to the control of electrical motors, and more specifically to methods and apparatuses for more effectively controlling the torque in a multiple phase brushless sensorless motor with a pulse width modulated (PWM) driving signal.
Electric motors may be used in a variety of devices, including disc drives and video cassette recorders (VCRs). Applications, such as these, tend to include a multiple phase brushless sensorless spindle motor that is controlled with a motor controller circuit. A motor controller circuit typically supplies a drive signal directly to the phase coils within the motor to operate the motor.
FIG. 1 illustrates a conventional disc drive 10 having a housing 12 that contains most of the disc drive components. A plurality of information storage discs 14 are journaled about a spindle motor assembly 16, having a spindle motor 18. A rotary actuator 20 carries a plurality of head arms 22, each having at least one associated read/write head 24 adapted for transferring information between the information storage discs 14 and an external computer system. A voice coil motor 26 provides precise rotary movement to rotary actuator 20 to accurately position heads 24. Thus, the combined motions of rotary actuator 20 and spindle motor assembly 16 allow the heads 24 to randomly access any segment of the discs 14. Control of voice coil motor 26 is accomplished through a voice coil motor controller 28. Control of spindle motor assembly 16 is accomplished through a motor controller 30. A disc I/O controller 32 is provided for transferring information to an external computer system through data port 34.
FIG. 2 further illustrates the connections between motor controller 30 and motor 18. As shown, motor controller 30 outputs a drive signal onto a drive signal line 36, for each phase, which cause motor 18 to operate. While operating, motor 18 outputs a back-EMF signal to motor controller 30 through a feedback line 38.
As known in the art, the drive signals applied to the coils of the motor may take on different waveforms, depending upon the system and the desired operation. Two common types of driving signals are linear and digital driving signals. Linear driving signals tend to have waveforms that are fairly continuous in nature, such as a direct current (DC) signal. Digital driving signals tend to have waveforms that are switched on and off over time, such as a digital pulse train. Pulse width modulation (PWM) is one example of a scheme to drive an electric motor using a digital pulse train. For instance, commonly assigned U.S. Pat. No. 4,972,130 issued Nov. 20, 1990 discloses a particular system that uses PWM driving circuits for driving the coils of a motor.
A typical objective in either a linear or a digital motor control system is to establish and maintain the operation of the motor as required for the application. For example, in a disc drive 10 the rotational speed of motor 18 may be held substantially constant, for a given load, by applying drive signals that supply a constant current to the coils so as to maintain a substantially constant torque.
In order to cause the desired torque in the motor, brushless motors typically require a motor controller capable of selectively connecting and disconnecting (i.e., commutating), each of the motor's coils to and from the driving signals at particular times. Calculating the proper commutation time usually requires determining, or monitoring, the location of the motor's rotor with regard to the coils. This may be accomplished, for example, by including sensors that relate such information to the motor controller circuit, or by evaluating a back-EMF signal generated in one or more of the coils within the motor. For sensorless motors, the back-EMF signal may be fed-back to the motor controller to determine the commutation time along with the difference (i.e., error) between the actual and desired rotational speeds. Such techniques are known to those skilled in the art, and include for instance, the methods and apparatuses disclosed in commonly assigned U.S. Pat. No. 5,317,243 issued May 31, 1994, U.S. Pat. No. 5,306,988 issued Apr. 26, 1994, U.S. Pat. No. 5,223,772 issued Jun. 29, 1993, and U.S. Pat. No. 5,221,881 issued Jun. 22, 1993, each of which are incorporated herein by reference.
FIG. 3 illustrates the basic shape of a back-EMF signal 40 generated by a three phase motor. As shown, back-EMF signal 40 is essentially a combination of three BEMF phase signals 42a, 42b and 42c that are superimposed over one another. BEMF phase signals 42a-c are sinusoidal in shape and 120.degree. out of phase as plotted with respect to angle .alpha.. BEMF phase signal 42b is shown as being zero volts at an angle .alpha..sub.1, 44, and BEMF phase signal 42c is shown as being zero volts at angle .alpha..sub.2 46. As shown, BEMF phase signals 42a and 42b are shown as crossing one another at angle .alpha..sub.3 48, BEMF phase signals 42a and 42c are shown as crossing one another at angle .alpha..sub.4 50, and BEMF phase signals 42b and 42c are shown as crossing one another at angle .alpha..sub.5 52. Typically, angles 48, 50 and 52 are referred to as commutation points.
Also shown in FIG. 3, there is a phase B vector 54 extending in the positive direction from zero volts to BEMF phase signal 42b, and a phase C vector 56 extending in the negative direction from zero volts to BEMF phase signal 42c. Vectors 54 and 56 represent the magnitude of the back-EMF for a given angle .alpha.. As known in the art, the torque created for each phase will be equal to the current flowing through the phase coil multiplied by the torque constant K.sub.T for the phase at that angle. The total effective torque applied to the motor will therefore equal the sum of the vectors, as shown below: EQU T=(.vertline.K.sub.T.sbsb.B(.alpha.) .vertline.+.vertline.K.sub.T.sbsb.C(.alpha.) .vertline.)I.sub.x
Typically, to drive the motor in a given direction, the motor is driven with a current in the direction that provides for a positive total torque. It is known in the art that to achieve maximum efficiency the commutation should be performed when the BEMF on the two phases is equal.
Unfortunately, a torque ripple may be introduced into the motor during commutation. Torque ripple can produce jitter in the motor and possibly an accompanying, acoustical noise. Torque ripple can typically be found in both linear and PWM systems because of the torque fluctuations occurring during commutation of phases due to the abrupt decay of the current in one coil and the relatively slower rise of the same in the next energized coil. The effects of torque ripple, such as introducing jitter in the system, are well known to those skilled in the art. For instance, commonly assigned U.S. Pat. No. 5,191,269 issued Mar. 2, 1993, addresses such problems in a linear system by disclosing circuitry that minimizes torque ripple in a linearly driven motor.
It is therefore the goal of many systems to maximize the torque, while minimizing the torque ripple. In theory, it is possible to design an optimal sinusoidal (linear) or pseudo-sinusoidal (digital) driving circuit wherein each of the BEMF phase signals is in phase with its respective driving signal's current. In such a system the power flow (energy) would theoretically be a constant, in accord with the following equation: EQU sin.sup.2 (.omega.t)+sin.sup.2 (.omega.t+120.degree.)+sin.sup.2 (.omega.t+240.degree.)=1
Thus, in principle such a system would yield zero torque ripple.
In practice, however, it is often very difficult to design such a digital system and also very expensive in terms of the quantity and quality of circuit components. For example, the motor controller circuitry would be required to maintain three different waveforms in synchronization which may require three separate supplies along with PLLs, or some form of digital circuitry with look-up tables, etc. Such a circuit, when embodied in an integrated circuit package may, for example, require anywhere from 3000 to 5000 logic gates.
In view of the foregoing, what is desired are cost effective methods and apparatuses for driving an electric motor with a PWM waveform wherein the total torque applied to the motor may be better controlled and torque ripple further reduced.